u/Actual_Oil_6770

I've been struggling with part of my data analysis for my thesis, I am attempting to see if the relationship between 2 variables changes (disappears) when considering a third variable, which I hypothesise causes both. My thesis mentor has repeatedly explained that, unless I can make a theoretical claim of causation, I can only perform an analysis as if this third variable is a mediator, but honestly by now I've lost the plot and am not sure how to proceed. I predominantly work through JASP as I'm able to download it onto my PC, but if need be I can access R or SPSS.
TL:DR Can you teach me how I'm supposed to test how a relationship between two (continuous) variables changes when checking for the influence of a 3rd (continuous) one?